Two stage process for producing purified gas

ABSTRACT

The present invention provides a process for producing purified gas from a feed gas stream comprising methane and gaseous contaminants, the process comprising the steps of: 1) providing the feed gas stream; 2) cooling the feed gas stream to a temperature at which at least part of the feed gas stream is present in the liquid phase; 3) separating the cooled feed gas stream by means of cryogenic distillation into a bottom stream rich in contaminants and lean in methane, and into a top stream rich in methane and lean in gaseous contaminants, in which the bottom stream contains between 0.5 and 15% of the methane present in the feed gas stream; 4) cooling the stream rich in methane to a temperature at which solid and/or liquid phase contaminants are formed; 5) introducing the cooled stream of step 4) into a gas/liquid/solids separation vessel, and 6) removing from the gas/liquid/solids separation vessel the purified gas and a stream rich in contaminants.

The invention relates to a process for producing purified gas from a feed gas stream comprising methane and gaseous contaminants. The invention further relates to a device to carry out the process, as well as to products made according to the process.

The removal of acidic contaminants, especially carbon dioxide and/or hydrogen sulphide, from methane containing gas streams has been described in a number of publications.

In WO 2004/070297 a process for removing contaminants from a natural gas stream has been described. In a first step, water is removed from the feed gas stream. This is especially done by cooling the feed gas stream resulting in methane hydrate formation, followed by separation of the hydrates. Further cooling resulted in the formation of solid acidic contaminants. After separation of the solid acidic contaminants a cleaned natural gas stream was obtained. It is preferred to convert the solid contaminants into a liquid by heating the solids. When high amounts of gaseous contaminants are present, only a part of the contaminants will be removed, and extensive further purification of the methane rich stream is required.

In WO 03/062725 a process is described for the removal of freezable species from a natural gas stream by cooling a natural gas stream to form a slurry of solid acidic contaminants in compressed liquefied natural gas. The solids are separated from the liquid by means of cyclone. It will be clear that a complete separation of the liquid from the solids is not easily achieved.

In WO 2007/030888 a process is described similar to the process described in WO 2004/070297, followed by washing the cleaned natural gas stream with methanol.

In U.S. Pat. No. 4,533,372 a cryogenic process is described for the removal of carbon dioxide and/or natural gas by treating the feed stream in a distillation zone and a controlled freezing zone.

In U.S. Pat. No. 3,398,544 the removal of gaseous contaminants from a natural gas stream is described by cooling to liquefy the stream and partly solidification of the stream, followed by expansion and separation of the cleaned gas stream and the solids.

There is still a need for a further improved process to produce a purified gas from a methane comprising feed gas stream. It is especially desired to design a relatively simple process, resulting in a methane rich stream which is almost pure methane and a contaminants stream that only contains a minimum amount of methane. Such a separation is difficult to achieve in a one-step process.

It has now been found that a purified gas can be produced from a feed gas stream comprising methane and gaseous contaminants in a two stage process. The first stage is a cryogenic distillation process in which the bulk of the gaseous contaminants is removed from the feed gas stream, followed by further cooling, for example by means of expansion, of the cleaned feed gas stream, leading to a solid and/or liquid contaminants stream, and a relatively pure methane stream. Optionally, a further cryogenic distillation stage can be included which yields a liquid methane overhead product with virtually no contaminants present and makes this process an integral part of an LNG production process.

Thus, the present invention provides a process for producing purified gas from a feed gas stream comprising methane and gaseous contaminants, the process comprising the steps of:

-   1) providing the feed gas stream; -   2) cooling the feed gas stream to a temperature at which at least     part of the feed gas stream is present in the liquid phase; -   3) separating the cooled feed gas stream by means of cryogenic     distillation into a bottom stream rich in contaminants and lean in     methane, and into a top stream rich in methane and lean in gaseous     contaminants, in which the bottom stream contains between 0.5 and     15% of the methane present in the feed gas stream; -   4) cooling the stream rich in methane to a temperature at which     solid and/or liquid phase contaminants are formed; -   5) introducing the cooled stream of step 4) into a gas/liquid/solids     separation vessel, and -   6) removing from the gas/liquid/solids separation vessel the     purified gas and a stream rich in contaminants.

The process according to the present invention provides an elegant way to produce purified gas. It actually only comprises the cooling of a feed gas stream, followed by cryogenic distillation, and a further purification step of the obtained methane enriched stream, by separating contaminants from the methane enriched stream as a solids and/or liquid stream. The two stage purification process results in a relatively very pure methane stream, while also the contaminants stream is relatively pure and only comprises a small amount of methane. Optionally a further cryogenic distillation step may be added to remove the remaining contaminants. Thus a methane stream is obtained that allows immediate production of LNG.

The feed gas stream containing methane and gaseous contaminants may be any methane containing gas, for instance from natural sources as natural gas, associated gas, coal bed methane or from industrial sources as refinery streams or synthetic sources as Fischer-Tropsch streams or from biological sources as anaerobic waste or manure fermentation. Gas streams, such as natural gas streams, may become available at a temperature of from −5 to 150° C. and a pressure of from 10 to 700 bar, suitably from 20 to 200 bar. The feed gas stream comprises besides methane suitably carbon dioxide and optionally hydrogen sulphide as acidic contaminants. The amount of the hydrocarbon fraction in the feed gas stream is suitably from 10 to 85 mol %, preferably from 25 to 80 mol %, based on the total gas stream. The hydrocarbon fraction in the natural gas stream may suitably contain from 0 to 20 mol %, suitably from 0.1 to 10 mol %, of C2-C6 compounds. The gas stream may also comprise up to 20 mol %, suitably from 0.1 to 10 mol % of nitrogen, based on the total gas stream.

The amount of methane present may vary over a wide range, e.g. from 3 to 90 vol %, preferably 5 and 80 vol % methane, more preferably between 10 and 90 vol %

The acidic contaminants in the feedstream are especially carbon dioxide and hydrogen sulphide, although also carbonyl sulphide (COS), carbon disulphide (CS2), mercaptans, sulphides and aromatic sulphur compounds may be present. Beside acidic contaminants, also inerts may be present, for instance nitrogen and noble gases as argon and helium. The amount of acidic contaminants present in the feed gas may vary over a wide range. The amount of hydrogen sulphide in the gas stream containing methane, if present, is suitably in the range of from 5 to 40 volume % of the gas stream, preferably from 20 to 35 volume %. The amount of carbon dioxide is in the range of from 10 to 90 vol %, preferably from 20 to 75 vol %, based on the total gas stream. It is observed that the present process is especially suitable for gas streams comprising large amounts of contaminants, e.g. 10 vol % or more, suitably between 15 and 90 vol %, and is especially suitable for gas stream comprising carbon dioxide as contaminant.

The amount of C₂+ hydrocarbons in the feed gas may vary over a large range. Suitably the amount of C₂+ hydrocarbons is between 0.1 and 25 vol %. Preferably there is in the feed gas between 0.2 and 22 vol % of C₂-C₈ hydrocarbons, more preferably between 0.3 and 18 vol % of C₂-C₄ hydrocarbons, especially between 0.5 and 15 vol % of ethane. Very suitably the feed gas is natural gas, associated gas, coal bed methane gas or biogas comprising acidic contaminants and C₂+-hydrocarbons. The term C₂+-hydrocarbons refers the ethane and higher hydrocarbons. The hydrocarbons comprise in principle all hydrocarbon compounds. Especially paraffins and monocyclic aromatic compounds may be present in the feed gas stream.

The raw feed gas stream may be pre-treated for partial or complete removal of water and optionally some heavy hydrocarbons. This can for instance be done by means of a pre-cooling cycle, against an external cooling loop, a cold internal process stream, or a cold LNG stream. Water may also be removed by means of pre-treatment with molecular sieves, e.g. zeolites, aluminium oxide or silica gel or other drying agents. Water may also be removed by means washing with glycol, MEG, DEG or TEG, or glycerol. Other processes for forming methane hydrates or for drying natural gas are also possible. The amount of water in the gas feed stream is suitably less than 1 vol %, preferably less than 0.1 vol %, more preferably less than 0.01 vol %. Water may also be removed by hydrate formation in the way as described in WO2004/070297. Suitably, water is removed until the amount of water in the natural gas stream comprises at most 50 ppmw, preferably at most 20 ppmw, more preferably at most 1 ppmw of water, based on the total feed gas stream.

The above-mentioned treatment step for water removal may also be applied prior to step (5), i.e. water may be removed from the stream rich in methane prior to introducing the stream rich in methane into the gas/liquid/solids separation vessel.

The amount of acidic contaminants that is removed by the process of the invention will depend on a number of factors. In practice, when using optimum conditions, at least 70 vol % (based on total acidic contaminants in the feed gas) of acidic contaminants will be removed, preferably at least 80 vol %, more preferably at least 90 vol %. The amount of methane that will remain in the contaminants streams, when using optimum conditions, will be between 1 and 15 vol % based on methane present in the feed gas stream.

Suitably the feed gas stream has a temperature of from −5 to 150° C. and a pressure of from 10 to 700 bara, suitably from 20 to 200 bara.

The cooling of the feed gas stream is suitably done by heat exchange against a cold fluidum, especially an external refrigerant, e.g. a propane cycle, an ethane/propane cascade or a mixed refrigerant cycle, or an internal process loop, suitably a carbon dioxide or hydrogen sulphide stream, a cold methane enriched stream or a cold LNG stream. In a preferred embodiment, the bottom stream obtained in the gas/liquid/solid separation vessel, optionally after liquefaction, may be used as an internal cooling stream.

In a preferred embodiment, additional cooling of the feed gas stream is done by nearly isentropic expansion of the feed gas stream, especially by means of an expander, preferably a turbo expander or laval nozzle. In another preferred embodiment, additional cooling of the feed gas stream is done by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially over a Joule-Thomson valve.

Suitably the feed gas stream is cooled to a temperature between −10 and −50° C., preferably between −20 and −40° C. before performing the cryogenic distillation step.

The cryogenic distillation is suitable performed in a cryogenic distillation column. Such columns are known in the art. Suitably the bottom temperature of the cryogenic distillation section is between −15 and 35° C., preferably between −5 and 30° C. A reboiler may be present to supply heat to the column. Suitably the top temperature of the cryogenic distillation section is between −70 and −40° C., preferably between −60 and −30° C. In the top of the cryogenic distillation column a condenser may be present, to introduce cold into the column.

Preferably the cryogenic distillation is carried out in such a way that the amount of gaseous contaminants in the top stream of the cryogenic distillation step contains between 5 and 40% of the gaseous contaminants present in the feed gas stream, preferably between 10 and 25%. Further, the bottom stream of the cryogenic distillation step contains between 0.5 and 10% of the methane present in the feed gas stream, preferably between 1 and 5%. Using an optimum design and optimum process conditions the methane loss in the process can be less than 3 vol %, or even less than 2 vol %. The remaining part of the bottom stream are the contaminants, and, when present, C2+, especially C3+, species. Usually most of the C3+ species will leave the cryogenic distillation unit via the bottom stream, the C2+ species will leave the column partly via the top, partly via the bottom.

Further cooling of the top stream rich in methane in step (4) is suitably done by expansion. Preferably the expansion is done by isenthalpic expansion, especially isenthalpic expansion over an orifice or a valve, in particular a Joule-Thomson valve. Alternatively, the expansion is done by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander, or a laval nozzle.

In another embodiment of the invention the cooling of the stream rich in methane in step (4) is done by heat exchange against a cold fluidum, especially an external refrigerant, e.g. a propane cycle, an ethane/propane cascade or a mixed refrigerant cycle, or an internal process loop, suitably a carbon dioxide of hydrogen sulphide stream, a cold methane stream, or a cold LNG stream.

At such cooling hydrocarbons may condense and such liquid condensate may be recovered before the gas stream is cooled further to the temperature at which acidic contaminants solidify. Preferably, the cooling stage of the natural gas stream comprises one or more expansion steps. For this purpose conventional equipment may be used. Conventional equipment includes turbo-expanders, so-called Joule-Thomson valves and venturi tubes. It is preferred to at least partly cool the gas stream over a turbo-expander, releasing energy. One advantageous effect of using the turbo-expander is that the energy that is released in the turbo-expander can suitably be used for compressing at least part of the purified gas. Since the stream of the purified gas is smaller than the feed gas stream now that acidic contaminants have been removed, the energy is suitably such that the purified gas may be compressed to an elevated pressure that makes it suitable for transport in a pipeline.

The cooling steps eventually lead to the desired temperature at which acidic contaminants solidify. However, since the feed gas stream also may comprise hydrocarbons other than methane it is preferred to cool the feed gas stream, suitably by expansion, to a temperature below the dew point of propane. In this way the vaporous gas stream will develop liquid hydrocarbons, including propane, which can subsequently be recovered easily from the vapour.

Combinations of expansion and cooling are also possible.

It will be clear that also a combination of the above-described cooling techniques may be used.

The solids collected at the bottom of the gas/liquid/solid separation vessel may be removed by methods known in the art. In a very suitable way to remove the solids at least part the solid contaminants obtained in step (6) are liquefied by heat exchange against a suitable heating stream. The heating stream may be an external heating stream, but is preferably an internal process stream, e.g. the dehydrated feed gas stream. In a preferred embodiment only solids are obtained, similar to the process as described in WO 2004/070297.

The methane rich stream leaving the gas/liquid/solids separation vessel usually contains at least 90 vol % methane, preferably at least 95 vol %. Using an optimum design and under optimum process conditions the methane content can be over 97 vol %, or even more than 98 vol %. Thus, the two stage process of the present invention is a very efficient separation process as most of the gaseous contaminants in the feed stream are removed.

The contaminants stream leaving the gas/solids separation vessel usually contains less than 10 wt % methane, preferably less than 5 wt %. Using an optimum design and optimum process conditions the methane loss in the process can be less than 3 vol %, or even less than 2 vol %.

In the event that the feed gas is natural gas, the purified natural gas can be processed further in known manners, for example by catalytic or non-catalytic combustion to produce synthesis gas, to generate electricity, heat or power, or for the production of liquefied natural gas (LNG), or for residential use.

Further process steps may be required in order to reach LNG specifications. In this case, preferably the methane enriched gaseous phase is further purified in a second cryogenic distillation process. The cryogenic distillation column is known in the art. Suitably the bottom temperature of the second cryogenic distillation section is between −30 and 10° C., preferably between −10 and 5° C. A reboiler may be present to supply heat to the column. Suitably the top temperature of the second cryogenic distillation section is between −110 and −80° C., preferably between −100 and −90° C. In the top of the second cryogenic distillation column a condenser may be present, to provide reflux and a liquefied (LNG) product.

As an alternative, further purification of the methane enriched gaseous phase may be accomplished by absorption with a suitable absorption liquid. Suitable absorbing liquids may comprise chemical solvents or physical solvents or mixtures thereof.

A preferred absorbing liquid comprises a chemical solvent and/or a physical solvent, suitably as an aqueous solution.

Suitable chemical solvents are primary, secondary and/or tertiary amines, including sterically hindered amines.

A preferred chemical solvent comprises a secondary or tertiary amine, preferably an amine compound derived from ethanolamine, more especially DIPA, DEA, MMEA (monomethyl-ethanolamine), MDEA (methyldiethanolamine) TEA (triethanolamine), or DEMEA (diethyl-monoethanolamine), preferably DIPA or MDEA. It is believed that these chemical solvents react with acidic compounds such as CO2 and H2S.

Suitable physical solvents include tetramethylene sulphone (sulpholane) and derivatives, amides of aliphatic carboxylic acids, N-alkyl pyrrolidone, in particular N-methyl pyrrolidine, N-alkyl piperidones, in particular N-methyl piperidone, methanol, ethanol, ethylene glycol, polyethylene glycols, mono- or di(C1-C4)alkyl ethers of ethylene glycol or polyethylene glycols, suitably having a molecular weight from 50 to 800, and mixtures thereof. The preferred physical solvent is sulfolane. It is believed that CO2 and/or H2S are taken up in the physical solvent and thereby removed.

Other treatments of the methane enriched gaseous phase may include a further compression, when the purified gas is wanted at a higher pressure. If the amounts of acidic contaminants in the purified gas are undesirably high, the purified gas may be subjected to one or more repetitions of the present process.

It is an advantage of the present process enables purification of natural gas comprising substantial amounts of acidic contaminants, resulting in purified natural gas comprising low levels of contaminants, especially of sulphur contaminants. The production of LNG from such natural gas, which would be very difficult if not impossible by conventional processes, is made possible. Thus, the invention also provides LNG obtained from liquefying purified natural gas obtained by the process. The LNG thus-obtained typically has very low concentrations of contaminants other than natural gas.

Since it is easier to transport liquids than to transport solids, it is preferred to melt at least partly the solid contaminants in the stream rich in contaminants. Therefore, it has been proposed to heat at least a part of the stream rich in contaminants to cause melting of the solid contaminants, thereby yielding a heated liquid contaminant-rich stream that is withdrawn from the bottom of the vessel, suitably by pumping.

Thus, in one embodiment where melting of solid contaminants is desired, the process further comprises the steps of:

-   (7) introducing the stream rich in contaminants into the     intermediate or bottom part or both of the gas/liquid/solids     separation vessel to obtain a diluted slurry of acidic contaminants; -   (8) introducing the diluted slurry of acidic contaminants via an     eductor into a heat exchanger in which solid acidic contaminant     present in the diluted slurry of contaminants is melted into liquid     acidic contaminant, wherein the heat exchanger is positioned outside     the separation vessel, and eductor is arranged inside or outside the     separation device or partly inside and outside the separation     vessel; -   (9) introducing part or all of the liquid contaminant obtained in     step 8 into a gas-liquid separator, wherein the gas-liquid separator     is preferably the bottom part of the gas/liquid/solids separation     vessel; -   (10) introducing part or all of the liquid contaminant obtained in     step 9 into the gas/liquid/solid separation vessel as described     above; -   (11) removing from the gas-liquid separator a stream of liquid     acidic contaminant and -   (12) separating the stream of liquid contaminant obtained in step 11     into a liquid product stream and a recirculation stream which is     used as a motive fluid in the eductor.

In this solid-melting embodiment, a continuously moving slurry phase is obtained, minimizing the risk of any blockages in the cryogenic separation vessel or in the pipelines and other pieces of equipment. Further, when a fully liquid stream is withdrawn from the heat exchanger, the absence of solid contaminant reduces the risk of blockages or erosion in subsequent pipelines or other equipment, and no damages will occur to any devices having moving parts, such as pumps. Moreover, when a pure liquid stream is withdrawn from the heat exchanger, a relatively cold liquid stream is obtained, thus minimizing the heat requirements of the separation device, and maintaining a high amount of exchangeable cold in the product stream.

Eductors, also referred to as siphons, exhausters, ejectors or jet pumps, are as such well-known and have extensively been described in the prior art. Reference herein to an eductor is to a device to pump produced solid and liquid CO2 slurry from the separator to the heat exchanger. The eductor is suitably designed for use in operations in which the head pumped against is low and is less than the head of the fluid used for pumping. Suitably, the eductor is a liquid jet pump. For a description of suitable eductors, also referred to as ejectors or jet pumps, reference is made to Perry's Handbook for Chemical Engineering, 8th edition, chapter 10.2. In accordance with the present invention any type of eductor can be used. Also a configuration may be used in which multiple eductors are uses.

Preferably, the eductor is arranged inside the separation device or partly inside and outside the separation device.

Suitably, a housing can be positioned around the eductor, enabling the eductor to be removed from the separation device. Such a housing can, for instance, be a vessel like containment, e.g. a pipe, that can be isolated from the process through valves.

In another embodiment of the present invention the eductor is arranged outside the separation device. Such an embodiment can be useful in situations in which the eductor in use needs to be repaired or replaced.

The eductor can be of such a size that it fits completely in the separation device or it may cover the entire diameter of the separation device, usually a vessel. However, it may also extend at two locations through the internal wall of the separation device.

In an alternative embodiment to melt solid contaminants, the process comprises the steps of:

-   (a) providing heat to the stream rich in contaminants to melt at     least part of the solid contaminants, yielding a heated     contaminant-rich stream; -   (b) withdrawing the heated contaminant-rich stream from the vessel; -   (c) reheating at least a part of the heated contaminant-rich stream     to form a reheated recycle stream; and -   (d) recycling at least a part of the reheated recycle stream to the     vessel.

The reheated recycle stream is recycled to the gas/liquid/solids separation vessel, to provide heat to the solid and/or liquid contaminants to melt at least part of the solid acidic contaminants. In this way the benefits of direct heat exchange are obtained whilst no alien species are introduced into the mixture. Further, the melting step can be carried out also when no condensates are available. Moreover, there is no need to provide for a complex heat exchanger in the lower part of the gas/liquid/solids separation vessel.

The recycle of part of the reheated recycle stream is intended to melt at least part of the solid acidic contaminants in the vessel so that blocking is prevented and removal of the acidic contaminants is facilitated. Preferably, the heat that is being provided by the recycled reheated recycle stream is such that it causes the melting of all solid acidic contaminants. The skilled person may achieve this by selecting the desired temperature of the reheated recycle stream and/or the amount of reheated recycle stream. Therefore, the part of the contaminant-rich stream that is reheated to form the reheated recycle stream is preferably heated to form a liquid stream, more preferably without any solid acidic contaminant. Suitably the heating up is done to a temperature well above the melting point of the solid acidic contaminants, such as at least 5° C. above the highest melting point. The heat of the relatively warm liquid will melt at least part of the solid acidic contaminants in the vessel. It is even more preferred that the part of the heated contaminant-rich stream that is reheated to form the reheated recycle stream is heated to such a temperature that the stream becomes at least partly vaporous. Not only will more energy be recycled to the vessel so that the melting of solid acidic contaminants is conducted more smoothly, but also any light hydrocarbon that may be entrained in the heated contaminant-rich stream will be freed up and can be included in the purified hydrocarbon gas that is withdrawn from the vessel. In this way the recovery of purified hydrocarbon gas is enhanced.

In the event that the contaminant-rich stream mainly comprises carbon dioxide and is therefore a CO2-rich stream, preferably CO2-rich stream is further pressurised and injected into a subterranean formation, preferably for use in enhanced oil recovery or for storage into an aquifer reservoir or for storage into an empty oil reservoir. It is an advantage that a liquid CO2-rich stream is obtained, as this liquid stream requires less compression equipment to be injected into a subterranean formation. Preferably, at least 90%, more preferably at least 95% and most preferably at least 98% of the solid acidic contaminants are melted. In this way a liquid stream of contaminants is obtained, which can be easily transported further.

The invention further relates to a plant for carrying out the process as described above, as well as to purified natural gas obtained by a process as described above. More especially, the invention also concerns a process for liquefying natural gas comprising purifying the natural gas as described above, followed by liquefying the natural gas by methods known in the art.

The process of the present invention is usually carried out in a continuous mode.

The invention is further explained by means of FIGS. 1 and 2. It will be understood that the Figures exemplify possible ways to apply the process and that the Figures are not limiting. In FIG. 1, a dry feed gas stream 1 (water content 20 ppmw., 70 bara, 30 wt % methane, 70 wt % carbon dioxide, 25° C.) is cooled down in heat exchanger 2 (this heat exchanger in reality represents a sequence of expansion, cooling via cold integration and cooling via external refrigeration) to a temperature of −15 to −35° C. The cooled stream is introduced in cryogenic distillation column 3. A gaseous top stream 4 is removed from the column and cooled down in condenser heat exchanger 5 to −40 to −55° C. Part of the stream is reintroduced as reflux into the distillation column via line 6. A liquid bottom stream 7 is removed from the distillation column. Part of the stream is heated in reboiler heat exchanger 8 and reintroduced in the column via line 9. The remaining part, comprising about 96 wt % carbon dioxide is removed from the process via line 10. The remaining stream from heat exchanger 5 is removed via line 11 and cooled down further, first in a heat exchanger (not shown), and subsequently over expansion valve 12. The stream is then introduced in gas/liquid/solids separation vessel 13. The stream comprises between 20 and 40 wt % carbon dioxide and between 60 and 80 wt % methane. A top stream 14 is removed from the gas/solid separator 14. This stream comprises 90 to 95 wt % methane. The solids in separator 13 are partly melted, for example by means of heating coil 16, and a slurry is removed form the separator via line 15. The slurry comprises about 98 wt % carbon dioxide. The heat in coil 16 may be provided by the warm feed gas stream 1.

In FIG. 2, a preferred embodiment of the process depicted in FIG. 1 is shown. The numbers and description of FIG. 1 apply to FIG. 2 also. In FIG. 2, a dry raw feed gas stream comprising methane and carbon dioxide is led via line 16 a to expander 25, where it is expanded. The expanded feed gas stream is cooled in heat exchanger 24 and the resulting cooled stream is led to an LNG heat exchanger 23. Resulting further cooled stream la comprising methane and carbon dioxide is cooled down in heat exchanger 2 and processed as described in FIG. 1. The second heat exchanger to cool down stream 11 is depicted here as 35. Stream 18 comprising methane and carbon dioxide may be subjected to further purification such as a second cryogenic distillation in a second distillation column (not shown). A third stream 20 comprising methane and carbon dioxide emanating from the LNG heat exchanger 23 is combined as stream 21 with the stream enriched in carbon dioxide and led from the process via line 22. In the heat exchanger 24 solid contaminant present is melted into liquid phase contaminant. Part of this liquid phase contaminant is passed via a conduit 26 b as a diluted slurry of contaminants to an intermediate position of separation vessel 13, whereas the main part of liquid phase contaminant is introduced into the bottom part of the separation vessel 13 by means of a conduit 26 a. The diluted slurry of contaminants is directed towards the top opening of an eductor 34. In the eductor 34 the diluted slurry is used as a suction fluid and via the eductor 34 it is passed into a heat exchanger 24 via a conduit 27. Liquid phase contaminant is subsequently withdrawn from the separation vessel 13 by means of a conduit 15 using a pump 33. Part of the withdrawn liquid phase contaminant is recovered as a product stream and part of said liquid phase contaminant is recycled via a conduit 15 a to the eductor 34. As an alternative, pump 33 may also be located in conduit 15 a. A funnel (not shown) is present to guide the slurry stream into the direction of conduit 27. Another part of said liquid phase contaminant is led via pump 19 to heat exchanger 34. In vessel 13 the solids and liquids, if any, will gather at the bottom of the vessel whereas the vapour, i.e., the purified gas is removed from the top of the vessel via a line 14. 

1. A process for producing purified gas from a feed gas stream comprising methane and gaseous contaminants, the process comprising the steps of: 1) providing the feed gas stream; 2) cooling the feed gas stream to a temperature at which at least part of the feed gas stream is present in the liquid phase; 3) separating the cooled feed gas stream by means of cryogenic distillation into a bottom stream rich in contaminants and lean in methane, and into a top stream rich in methane and lean in gaseous contaminants, in which the bottom stream contains between 0.5 and 15% of the methane present in the feed gas stream; 4) cooling the stream rich in methane to a temperature at which solid and/or liquid phase contaminants are formed; 5) introducing the cooled stream of step 4) into a gas/liquid/solids separation vessel, and 6) removing from the gas/liquid/solids separation vessel the purified gas and a stream rich in contaminants.
 2. A process according to claim 1, in which the feed gas contains between 15 and 90 vol % of carbon dioxide.
 3. A process according to claim 1, in which the feed gas contains between 5 and 55 vol % of hydrogen sulphide, and between 5 and 80 vol % of methane.
 4. A process according to claim 1, in which the feed gas stream has a temperature between −10 and 70° C., and a pressure between 20 and 150 bara.
 5. A process according to claim 1, in which the cooling of the feed gas stream is done by heat exchange against a cold fluidum, comprising an external refrigerant.
 6. A process according to claim 5, in which additional cooling of the feed gas stream is done by nearly isentropic expansion or in which additional cooling of the feed gas stream is done by isenthalpic expansion.
 7. A process according to claim 1, in which the feed gas stream is cooled to a temperature between −10 and −50° C.,
 8. A process according to claim 1, in which the bottom temperature of the cryogenic distillation section is between −5 and 30° C., or in which the top temperature of the cryogenic distillation section is between −60 and −30° C.
 9. A process according to claim 1, in which the top stream of the cryogenic distillation step contains between 10 and 25 wt % of the gaseous contaminants present in the feed gas stream or, in which the bottom stream of the cryogenic distillation step contains between 0.5 and 15 wt % of the methane present in the feed gas stream.
 10. A process according to claim 1, in which the cooling of the stream rich in methane in step (4) is done by isenthalpic expansion, or in which the cooling of the stream rich in methane in step (4) is done by nearly isentropic expansion.
 11. A process according to claim 1, in which the cooling of the stream rich in methane in step (4) is done by heat exchange against a cold fluidum, comprising an external refrigerant.
 12. A process according to claim 1, the process further comprises the steps of: (7) introducing the stream rich in contaminants into the intermediate or bottom part or both of the gas/liquid/solids separation vessel to obtain a diluted slurry of acidic contaminants; (8) introducing the diluted slurry of acidic contaminants via an eductor into a heat exchanger in which solid acidic contaminant present in the diluted slurry of contaminants is melted into liquid acidic contaminant, wherein the heat exchanger is positioned outside the separation vessel, and eductor is arranged inside or outside the separation device or partly inside and outside the separation vessel; (9) introducing at least a part of the liquid contaminant obtained in step 8 into a gas-liquid separator, wherein the gas-liquid separator is preferably the bottom part of the gas/liquid/solids separation vessel; (10) introducing part or all of the liquid contaminant obtained in step 9 into the gas/liquid/solid separation vessel as described above; (11) removing from the gas-liquid separator a stream of liquid acidic contaminant; and (12) separating the stream of liquid contaminant obtained in step 11 into a liquid product stream and a recirculation stream which is used as a motive fluid in the eductor.
 13. A process according to claim 1, the process further comprises the steps of: (a) providing heat to the stream rich in contaminants to melt at least part of the solid contaminants, yielding a heated contaminant-rich stream; (b) withdrawing the heated contaminant-rich stream from the vessel; (c) reheating at least a part of the heated contaminant-rich stream to form a reheated recycle stream; and (d) recycling at least a part of the reheated recycle stream to the vessel
 14. A process according to claim 1, wherein the feed gas stream is a natural gas stream and the purified gas is purified natural gas, the process further comprising the step of subjecting the purified natural gas to a second cryogenic distillation, followed by liquefying the natural gas to produce liquefied natural gas.
 15. (canceled) 